none of the old sith can take luke. and there isnt enough on them to say they can. everything with them is speculation. its just implied that marga ragnos was a great and powerful sith for his time. we have no idea how he manifested that power before he was a ghost. i would say that luke could rock him. but once again thats just spec. because ragnos could either have the power of coleman trebor or 3 times the emperor for all we know.